1. Field of the Invention
This invention relates to pigments and compositions containing such pigments, particularly fluoropolymer compositions, and their use in laser marking of substrates.
2. Introduction to the Invention
Polymers such as fluoropolymers are commonly used as insulating materials for substrates such as wire and cable. For such applications, the fluoropolymer generally surrounds a central wire or provides a jacket around one or more insulated wires. In order to identify particular wires, it is often necessary to mark the insulation or jacketing material with numbers, letters, or other indicia. Laser marking is one preferred method of marking, as it can provide a relatively permanent, highly legible mark on the surface of the insulation and does not produce mechanical damage or require good adhesion of an ink to the polymer.
Laser marking often uses a ultraviolet (UV) laser such as an excimer laser. Because fluoropolymers are transparent to UV radiation, it is generally necessary that a UV light absorbing compound or pigment be added to the fluoropolymer in order to produce marks. A commonly used additive is titanium dioxide (TiO2). When a laser is directed at the additive-containing polymer composition, the photosensitive TiO2 changes color as a result of the laser-induced reduction of Ti4+ (colorless) to Ti3+ (blue-black) in the TiO2 lattice. The use of TiO2 in a fluoropolymer is disclosed in U.S. Pat. Nos. 5,560,845 and 5,789,466 (both Birmingham et al.), which provide details on laser marking of pigmented melt-processible fluoropolymer substrates that use titanium dioxide pigment coated with organosilane. These documents rely on the organosilane to increase the dispersion of the TiO2 pigment, reducing the number of TiO2 agglomerates and increasing the quality of the mark. In the disclosed compositions, the organosilane is present in an amount from about 0.1 to about 5 weight percent based on the amount of the organosilane and the pigment.
We have now found that laser marks of improved contrast can be produced if the pigment is coated with or in close proximity to a synergist that contains a substituent that upon pyrolysis generates a black material, e.g. carbon, that provides a mark. Such pigments can be used in compositions that are exposed to excimer laser radiation, for example in insulation for wires and cables. Different types of synergists can be used to accommodate the different processing and use conditions of the polymer. In particular, we have found laser marks formed using the pigments of the invention and compositions comprising them retain good contrast under heat aging conditions.
In a first aspect this invention provides a colorless UV-absorbing pigment at least partially coated with a synergist having the formula
[Rm(SiOn)]pRxe2x80x2q,
wherein
(a) m is 1 to 3, n is 1 to 3, p is at least 1, and q is 0 to 3,
(b) at least one R or Rxe2x80x2 is a substituent that upon pyrolysis generates a black material suitable for providing a mark.
In second aspect, the invention provides a composition suitable for laser marking when exposed to radiation from an excimer laser, said composition comprising
(1) a fluoropolymer having a processing temperature Tp,
(2) 0.1 to 25% by weight of the composition of a colorless UV-absorbing pigment, and
(3) a synergist according to the first aspect of the invention, said synergist being (i) present at at least 10% by weight of the pigment present in the polymer composition, (ii) heat stable at a temperature of at least Tp, and (iii) in physical proximity with the pigment.
Particularly preferred are compositions in which the synergist and pigment are used in polytetrafluoroethylene (PTFE). Therefore, in a third aspect, this invention provides a composition suitable for laser marking when exposed to radiation from an excimer laser, said composition comprising
(1) polytetrafluoro ethylene,
(2) 0.1 to 50% by weight of the composition of a colorless UV-absorbing pigment, and
(3) a synergist having the formula
[Rm(SiOn)]pRxe2x80x2 q,
wherein
(a) m is 1 to 3, n is 1 to 3, p is at least 1, and q is 0 to 3, and
(b) at least one R or Rxe2x80x2 is a substituent that upon pyrolysis generates a black material suitable for providing a mark,
said synergist being (i) present at at least 0.1% by weight of the pigment present in the polymer composition, (ii) heat stable at a temperature of at least Tp, and (iii) in physical proximity with the pigment.
Pigments and compositions of the invention are particularly useful for insulating materials. Therefore, in a fourth aspect, this invention provides an insulated conductor which comprises
(A) an elongate wire, and
(B) an insulating layer surrounding said wire, said layer comprising a composition of the second aspect of the invention.